The invention relates to a voltage-dependent resistor having a ceramic sintered body based on a polycrystalline alkaline earth metal titanate which has been doped with a small quantity of metal oxide to produce N-type conductivity, electrodes being provided on opposed parallel surfaces.
The invention further relates to a method of producing such a resistor.
Voltage-dependent resistors (denoted varistors hereinafter) are resistors the electric conductivity of which increases to a very large extent with increasing voltage at a constant temperature. This behaviour can be approximated by the following formula: EQU U=C.I.beta.
wherein:
I=the current through the varistor in Amps. PA1 U=the voltage drop at the varistor in Volts PA1 C=a constant which depends on the geometry; it indicates the voltage at I=1 A. In practice it may assume values between 15 and some thousands. PA1 .beta.=the current index, coefficient of non-linearity or control factor; it depends on the material and is a measure of the slope of the current-versus-voltage characteristic. PA1 (a) sintering in a reducing atmosphere a polycrystalline alkaline earth metal titanate such as SpTiO.sub.3 with a metal oxide additive which functions as a doping material to produce a N-type conductivity at a temperature in the range from 1200.degree. to 1400.degree. C., preferably 1350.degree. C. PA1 (b) milling and mixing the sintered material obtained in accordance with step (a) with a lead germanate phase defined by the general formula (PbO).sub.x : (GeO.sub.2).sub.y in the molar ratio x:y=5:1 to 1:5 PA1 (c) pressing the mixture obtained in accordance with step (b) into preforms which are suitable for a resistor, PA1 (d) sintering in air the preform obtained in accordance with step (c), a thin insulating layer being formed during sintering from the lead germanate phase at the grain boundaries of the polycrystalline semiconducting alkaline earth metal titanate at a temperature in the range from 1050.degree. to 1350.degree. C., preferably 1200.degree. C. PA1 (e) Providing facing planes of the sintered body obtained in accordance with step (d) with metal electrodes. PA1 (a) sintering a mixture consisting of 98% by weight of SrTiO.sub.3 having an average grain size of 1.mu.m and 2% by weight of Bi.sub.2 WO.sub.6 at a temperature of 1350.degree. C. in a reducing atmosphere, PA1 (b) milling and mixing 80 to 90% of the sintered material obtained in accordance with step (a) and 10 to 20% by weight of lead germanate Pb.sub.5 Ge.sub.3 O.sub.11, PA1 (c) pressing the mixture obtained in accordance with step (b) into tablets having a diameter of 6 mm and a thickness of 0.55 mm PA1 (d) sintering the preforms obtained in accordance with step (c) at a temperature in the range from 1150.degree. to 1350.degree. C. in air and PA1 (e) providing facing planes of the sintered body obtained in accordance with step (d) with CrNi-Au electrodes by means of vapour deposition.
Preferably, the .beta. value must be as low as possible as at a low value of .beta. pronounced changes in the current will result in only small changes in the voltage at the varistor.
The C-value chosen depends on the purpose for which the varistor is used; it determines the limit of the voltage value, and preference is given to a ceramic sintered body for which it is possible to realise in a simple way C-values over a wide range.
This is particularly possible with sintered bodies in which the non-linearity of the resistance is based on properties which are inherent in the sintered body; the C-value can here be controlled.
For varistors comprising zinc oxide as the main component and, for example, bismuth, antimony, manganese, cobalt or chromium as additives, the voltage-dependence is based on, for example, the properties which are inherent in the sintered body. As previously mentioned, this is an advantage. These varistors have, however, the disadvantage that the use of volatile components is a requirement in the production of the sintered body. These volatile components evaporate at the high temperatures which are required during sintering of the green body for the desired varistor. Particularly the volatility of bismuth has a disturbing effect and it is therefore difficult to sinter the green body during large-scale production in such a way, without considerable loss, that varistors having uniform properties can be produced. For certain uses varistors on the basis of ZnO may have the further disadvantage that at a constant voltage the power which can be converted in these varistors is limited by an operating current which increases with increasing temperature and which leads to additional heating of the component.
U.S. Pat. No. 2,885,521 discloses a non-linear resistor based on ferroelectric barium titanate, in which doping materials which produce N-type conductivity (bismuth oxide, antimony oxide or arsenic oxide) are incorporated in the perowskite lattice of the titanate. This resistor does not acquire its properties as a voltage-dependent module until a semiconducting channel has been formed by dielectric breakdown in the resistor body after a voltage has been applied. This prior art material corresponds to a very large extent to a PTC resistor, the non-linearity of these resistors being produced by these ranges of different power behaviour. For these resistors the predicted C-values cannot be reproducibly obtained.